It is known that in Arctic environments, accumulation of ice is extreme during November to June, resulting in widespread sheet ice. During June to August arctic environment is known to have floe ice. It is now an established fact that the areas at the margin regions of the Arctic Ocean, have substantial reserves of hydrocarbons like oil and natural gas, often below relatively shallow waters.
Oil and natural gas exploration and production drilling in the Arctic waters, has shot up substantial problems in the past, mainly due to the fact that conventional offshore drilling and production platforms/equipments/systems are not adapted to withstand the substantial drift, caused by sheet ice and ice floe. The load acting on such structures, due to ice drift, leave such structures open to the catastrophic possibility of lateral shifting from its position, during drilling. Hence, such structures have been designed to act as very solid and heavy structures such as for example artificial islands, for undertaking drilling and production of hydrocarbons in Arctic environment. However, these structures have to be fixed by heavy anchoring on the sea bed to withstand lateral drifting of ice and can not be moved to other locations. Hence, these structures are then converted to production facility, which involves very expensive modifications in the remote and harsh locations.
The disadvantages as narrated in the preceding paragraph, triggered the need for movable drilling units for undertaking drilling operations in the Arctic environment. Of course, movable drilling vessels known in the art could not be deployed due to their incapability to withstand the heavy lateral drifting of ice. This has marked the popularity of designing movable structures such as gravity based structures for undertaking drilling and production operations in Arctic waters round the year, for recovery of hydrocarbons.
WO84/02151 attempts to disclose one such gravity based structure, as mentioned in the preceding paragraph, specifically designed for the Arctic environment. However, such structure comprising modular ballastic units involves a complex and bulky construction. That apart, it has to be selectively assembled to suite specific conditions and locations.
WO 02/092425 also discloses movable gravity based structure defining mating of a male and female coupling, but in addition to being bulky, such a structure is not suited to withstand the substantial lateral drift caused by ice, in Arctic environment.
WO 2006/086240 discloses a system for stabilizing a gravity based offshore structure which is movable, but this structure has the requirement for having a different substructure anchored to the sea bed for effective functioning of the gravity based structure. That apart, this structure cannot withstand heavy drifting of ice. Moreover, the top side structure cannot be moved without assistance from heavy crane vessels.
WO 2007/126477 discloses a Mobile Year Round Arctic drilling system comprising jack up legs through a hull for its lowering and elevation so that the drilling system can be lowered on the sea bed and also elevated from water on completion of drilling and thereafter moved to a different location. However, it has the disadvantage that the entire drilling system or a substantial portion of it needs to be relocated to a new site, leaving behind at least one foundation unit, for protecting well head. So, relocating such entire structure is cumbersome, time consuming and involves substantial cost. Also, the operation, naturally requiring mandatory penetration of the sea bed by at least one foundation unit is complicated and that apart, the system is heavily dependent on designing of jack up legs for its functioning and involves a complex and expensive design.
U.S. Pat. No. 4,126,011 describes a platform with a floatable top member which is adapted to be attached to a gravity based structure having a tower extension extending above the water surface. The top member has jack-up legs that can be attached to the tower.
A disadvantage of this system is that the jack-up legs are engaged at all times. This means that the legs are subject to ice influence. The jack-up legs will not be able to withstand the substantive forces from the ice and the platform will soon be in jeopardy.
The so-called mobile drilling systems referred to in the preceding paragraphs are hardly moveable and with one, possibly two, exceptions, requires the assistance of heavy crane vessels. They all have one problem or the other in respect of their suitability for application in drilling operations in Arctic waters, for recovery of hydrocarbons.
Accordingly there has been a desire for a mobile drilling system for Arctic environment, which has a simple construction, is economic, is technically effective to withstand heavy lateral ice drift, thereby enabling year round drilling in Arctic waters and has the possibility to be easily shifted from one location to the other without the assistance of other vessels, such as heavy crane vessels. There is also a need to provide protection for the well equipment above the seabed against ice drifts.
The present invention meets above long felt need and other associated needs.